Saturday, October 4, 2014

Dinosaur Arms to Bird Wings: It's All in the Wrist

One of the last niggling doubts about the link between dinosaurs and
birds may be settled by a new study that shows how bird wrists evolved
from those of their dinosaur predecessors.
The study, reported in the Journal PLOS Biology,
shows how nine dinosaurian wrist bones were reduced over millions of
years of evolution to just four wrist bones in modern day birds.
"This discovery clarifies how dinosaur arms became bird wings," said one of the study's authors, Dr Alexander Vargas of the University of Chile in Santiago.
"It shows that some bones fused, other bones disappeared, and one bone disappeared and then reappeared in evolution."
Skeletal similarities between theropod dinosaurs and birds
provide some of the strongest evidence showing how birds developed from
dinosaurs. But the evolution of straight dinosaur wrists into
hyperflexible wrists allowing birds to fold their wings when not flying,
has remained a point of contention between palaeontologists and some
developmental biologists.
Among the structures in question is a half-moon shaped wrist
bone called the semilunate which is found in dinosaurs and looks very
similar to a wrist bone also found in birds.
The semilunate originated as two separate dinosaur bones which
eventually fused into a single bone. However some developmental
biologists claim it evolved as a single bone in birds, and so isn't the
same bone as that found in dinosaurs.
To help settle the debate, Vargas and colleagues examined the
wrist bones of dinosaur fossils in the collections from several museums,
and compared them to new developmental data from seven different
species of modern birds.
"We developed a new technique called whole-mount
immunostaining, which allows us to observe skeleton development better
than ever before, including the expression of proteins inside embryonic
cartilage," said Vargas.Read more here